Output driver circuits are used to transmit signals out of an integrated circuit device. Differential output drivers are typically used to transmit serial data at high speeds with low voltage signal swings. For example, the data bus of the Universal Serial Bus (USB) uses differential signals to communicate data between USB devices. Per the USB 2.0 specification, high speed output drivers that desire to write data to the USB bus must provide 18 milliamps of current on the properly terminated data bus lines D+ and D−.
In such an arrangement, one issue relates to power management of the output driver of an integrated circuit adapted to communicate over the USB bus. A first conventional solution uses a P-type field effect transistor (PFET) switch in series with a current source to enable and disable an output. However, when the output-enable switch turns on, coupling through the large gate-drain capacitance of the current source disrupts the current source bias voltage. An alternate approach comprises a “dump” PFET in parallel with the output-enabled PFET. When the output is disabled, the current source is shorted to ground, wasting power. A disadvantage of the first conventional solution is that it always burns the current source power, even when not transmitting.
In another solution, shown in FIG. 1, a device comprises a plurality of blocks each dumping current. The current dumping occurs only briefly before and after a transmit operation. In this approach, when the dump path is enabled before a transmit operation, coupling through the large current source gate-drain capacitance significantly disrupts the PBIAS node. A correction amplifier senses the bias change, and sinks/sources current in response. A disadvantage of this approach is that it requires the design of the bias correction amplifier which consumes static current constantly.
As recognized by the present inventors, what is needed is an improved output driver that consumes a reduced amount of current, which would be useful various application including battery powered applications such as mobile computing or for bus-powered USB applications.
It is against this background that embodiments of the present invention have been developed.